In this type of reactor, the main vessel contains a cooling liquid in which the reactor core is immersed, the core consisting of juxtaposed fuel arrays and releasing heat produced by fission, the heat being transferred to the cooling liquid filling the vessel, which is generally liquid sodium.
The vessel is closed at its upper part by a thick slab, which is itself supported by the protective concrete wall of the reactor. The main vessel is generally fixed to the slab, whereby the assembly comprising the vessel, the components and the sodium which it contains is suspended from the slab via the upper part of the vessel, which is thus subjected to large strains.
It is therefore necessary to prevent the main vessel, in contact with the liquid sodium, from being raised to a high temperature or from undergoing temperature variations, during the operation of the reactor.
Furthermore, a partition with a symmetry of revolution, which is generally a double partition, separates the interior volume of the main vessel into two zones, one of which contains the reactor core. In this zone, the liquid sodium passing through the core from bottom to top leaves the core at a temperature of the order of 560.degree. C., whereas the sodium enters the core, in the lower part of the latter, at a temperature of the order of 400.degree. C.
For this reason, the sodium leaving the core and filling the first zone of the main vessel is referred to as "hot sodium", whereas the sodium entering the reactor core is called "cold sodium".
A series of heat exchangers referred to as "intermediate exchangers" are furthermore arranged inside the main vessel, so that the inlet for primary fluid in these exchangers is located inside the first zone containing the hot sodium, and so that the outlet for primary fluid emerges in the second zone created in the main vessel by the separating partition. Inside the intermediate exchanger, the sodium filling the vessel, referred to as "primary sodium", heats sodium referred to as "secondary sodium", which is sent to steam generators in which it causes the vaporization of the feed water. The primary sodium coming from the first zone has cooled by heating the secondary sodium, whereby the second zone of the main vessel receives cold sodium, which is sent from this zone, by virtue of a series of pumps immersed in the vessel, to the lower part of the reactor core, under the arrays, by virtue of the outlet pipes of the pumps, which are in communication with this part of the core.
To provide the cooling and keep the main vessel at a constant temperature, cold sodium is furthermore circulated in contact with the internal surface of the latter, at least in its upper part. To do this, at least two cylindrical shells are arranged inside the vessel and coaxially thereto.
The shell of larger diameter, or external shell, creates an annular space between its external surface and the internal surface of the main vessel, and this space is brought into communication with the lower part of the core, i.e., the zone into which the cold sodium is injected.
The two shells create between one another a second annular space, which is brought into communication with the second zone created in the vessel containing the cold sodium. These two annular spaces emerge, in their upper part, in a zone between the slab and the upper level of sodium in the vessel, which is filled with a blanket of inert gas protecting the slab.
Part of the cold sodium injected by the pumps is sent into the external annular passage and circulates therein, from bottom to top, in contact with the internal surface of the main vessel, which it cools and keeps at the temperature of the cold sodium. In the upper part of the external space, the sodium flows into the second annular space and moves down again, by gravity, into the second zone of the vessel, containing the cold sodium.
Various improvements to this device for cooling by the circulation of cold sodium have been envisaged, but, in all cases, the external shell is subjected to the pressure of the liquid sodium on its external surface, in its upper part where this pressure is not equilibrated by the pressure of the sodium contained in the second annular space, because of the difference in sodium level between these two spaces, making it possible for the sodium to flow from one space to the other.
In the same way, the lower part of the shell is subjected to a differential pressure due to the difference in height of the sodium columns in the two annular spaces.
It is therefore necessary to provide shells which are strong and hence of large thickness. The weight of the assembly supported by the main vessel is thereby increased accordingly.